This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Acquiring sufficient supplies of clean energy is presently a critical challenge. Sunlight is a plentiful carbon neutral source that may be harnessed to partially satisfy this need. While conventional single crystal inorganic semiconductor devices can convert more than twenty percent of received solar energy to electric power, their high cost and relatively complicated technology limit large scale industrial and household application. Accordingly, there is interest in developing solar cells using alternative materials.
Organic materials represent a class of alternative materials that may be applicable to the development of solar cells that are relatively simple and cost efficient. The opportunities for using organic photovoltaics (OPV) are considerable and could lead to low-cost, lightweight, ultrafast optoelectronic response, easily processable, large area, flexible solar cells. Hybrid devices incorporating both organic and inorganic materials can reap the advantages of both classes of materials.
Hybrid solar cells have been developed in the past decade as a promising alternative for traditional silicon-based solar cells. A wide band gap metal oxide, for example, titanium dioxide (TiO2), sensitized by an organic semiconductor, dye molecule, or quantum dots offers the promise of low-cost, large-area conversion of solar energy to electricity. However, the nano-scale morphology of such devices is an important element of their performance. For instance, a simple layered (planar heterojunction) donor-acceptor device structure yields cells with poor efficiency due to the limited interfacial area, charge carrier recombination, and overly thin layers necessitated by exciton diffusion distances (5-20 nm). Enlargement of the interfacial area is accomplished in dye-sensitized solar cells, in which a highly porous film of titanium dioxide nanoparticles is covered with a monolayer of a metal-organic sensitizer that absorbs visible light. Although energy conversion efficiencies can exceed ten percent in such devices, the necessity of a liquid electrolyte to accomplish regeneration of the oxidized dye usually calls for elaborate sealing techniques that have hindered commercialization. Further, conventional bulk heterojunction (BHJ) solar cells consist of randomly structured contact between the donor and acceptor layers; limitations of this disordered configuration include non-ideal domain length scales, charge trapping at bottlenecks, and dead-ends in the conducting pathways to the electrodes.
In contrast, highly ordered, vertically oriented, crystalline oxide semiconductor (such as TiO2) nanotube arrays fabricated by potentiostatic anodization provide excellent electron percolation pathways for direct charge transfer between interfaces. This material architecture offers a large internal surface area without a concurrent degradation of structural order. Further, this architecture offers the ability to influence the absorption and propagation of light through the architecture by precisely designing and controlling the architectural parameters including nanotube pore size, wall thickness, and length.
One approach for making inexpensive inorganic-organic hybrid photovoltaic (PV) cells is to fill nanostructured titania films with solid organic hole conductors such as conjugated polymers. These compounds can function as light-absorbing species and inject electrons into the conduction band of the n-type semiconductor, while at the same time they conduct the holes to the cathode. For example, oligothiophenes and polythiophenes (PT), in particular, have strong potential in the fields of electronics, sensors, solar cells, and displays because of their superior thermal and environmental stability as well as their interesting electronic properties. In particular, poly(3-hexylthiophene) (P3HT) is considered advantageous due to its large absorption coefficient (close to the maximum photon flux in the solar spectrum) and its high hole mobility of 0.1 cm2 Vs in its ordered, regioregular form, which is among the highest for polymeric semiconductors. Other types of conjugated polymers can also be used.
Nanotube films offer a distinct advantage over nanoparticle films in that they facilitate charge carrier transport. The electrons in particulate TiO2 films are more susceptible to loss at grain boundaries than those in nanotube TiO2 films. The relative roles of crystal structure and surface defects also must be taken into consideration in comparing TiO2 tubes and particles in the context of their interaction with polymers. In addition to the improved electron mobility associated with ordered metal oxide nanostructures, the hole mobility of the conjugated polymer may be enhanced in the direction normal to the substrate by infiltrating the polymer into a nanotube architecture as a result of alignment of the polymer chains along the walls of the pores. Compared to the more commonly used ruthenium-based dyes, conjugated polymers are relatively inexpensive as sensitizers. In films sensitized by molecular dyes, a thickness of nanostructured TiO2 film of at least 10 μm is necessary to harvest the maximal amount of incident photons. On other the hand, for a polymer with a high absorption coefficient such as P3HT, a film several hundred nanometers in thickness is sufficient to optimally harvest incident sunlight. Thinner films translate into shorter pathways for the charge carriers and, hence, less non-geminate recombination.
The infiltration of the polymer into the nanostructured metal oxide is of particular importance for optimizing the performance of these hybrid devices. Past efforts to develop solar cells using conjugated polymers have employed wet processing deposition techniques such as spin coating, dip coating, drop casting, doctor-blading, ink-jet printing, and screen printing. However, because polymers suffer a loss of conformational entropy when they are confined in a channel that has a radius less than their radius of gyration, filling the pores with a polymer has been thought to be a challenge due to the possibility of the polymer chains clogging the pores of the nanotubular electrode. These deficiencies may be avoided by producing oligothiophenes and polythiophene directly within a nanostructured architecture instead of using presynthesized polymer. Solventless direct deposition approaches such as plasma polymerization, laser-induced chemical vapor deposition, as well as X-rays, electrons, and ion-induced synthesis in ultra-high vacuum (UHV) conditions have also been attempted. However, these approaches, in general, have insufficient reaction specificity to generate reactive species without fragmentation of the monomer structure, resulting in defect incorporation in the final product.